Department of Mechanical Engineering
University PhD Dissertation Defense
Title: Microstructured Adhesives for Climbing Applications
Advisor: Prof. Mark Cutkosky
Time: Oct. 2nd, 2009, 2:30 pm (Refreshment served at 2:15pm)
Location: Mechanical Engineering Research Lab (MERL), Conference room (2nd floor, by the kitchen area)
Campus map: http://campus-map.stanford.edu/index.cfm?ID=02-660
Abstract:
Researchers seeking to expand the capabilities of mobile robots have begun looking to biological systems for inspiration. One particularly agile creature, the gecko lizard, is remarkably adept at maneuvering across both flat and vertical surfaces. Some species of gecko are even capable of climbing across inverted surfaces. The study of the gecko's adhesive system has informed the design of several synthetic adhesives in recent years. However, these adhesives generally fail to match the gecko adhesive's performance in one fashion or another. Many previous synthetic gecko adhesives are not reusable or they lack a method of control. Others do not have the ability to conform to surfaces with any roughness, or to distribute forces across the many thousands of fibers evenly. This presentation focuses on the design of a gecko-like adhesive system that achieves the level of performance necessary for implementation on a small-scale climbing robot.
A gecko's adhesive structure has a strong directional preference, allowing the animal a method to control the stickiness of its feet. When loaded from the tip of the toe towards the palm, the material exhibits high adhesion in both the shear and normal direction. However, when this load is released, or when the toe is loaded in a different direction, no adhesion is present. First, I will discuss the creation of a synthetic micro-structure that also displays a directional adhesive dependence. This begins with the presentation of a new lithographic process used to create asymmetric wedge-shaped cavities in a photo-sensitive epoxy at the scale of tens of microns. Elastomeric materials were cast into these molds to produce the synthetic micro structures. Analysis and laboratory testing of this material show its strong directional dependence. Its performance for robotic climbing applications was promising at small sample sizes when tested on smooth surfaces like glass. However, the material proved inadequate for climbing because its performance could not be scaled to areas greater than about 1 cm2, nor could it adhere to rough surfaces.
The gecko uses a multi-tiered hierarchy to insure that its millions of sub-micron sized spatulae make intimate contact with a surface regardless of its roughness. The gecko's system conforms across multiple length scales, distributing climbing forces and allowing rapid locomotion with seemingly minimal control effort. In the second portion of the talk, hierarchical suspensions for fibrillar adhesives are analyzed, and three iterations of a synthetic hierarchical adhesive design are detailed. The use of these hierarchical suspensions with modified wedge micro-structures similar to those mentioned above turned out to be successful enough to allow a mobile robot platform named Stickybot to climb multiple vertical surfaces ranging in roughness from glass to drywall. Data from large patches, well over 100 cm2, are also included. Discussion of these results and their implications for human climbing applications will conclude the presentation.